A ReviewBoard Extension that demonstrates how to:
- Register a ReviewUI for XML file attachments.
- Register a Mimetype Handler to create XML thumbnails for these attachments.

Currently:
- Both the ReviewUI / Mimetype Handler provide syntax / keyword highlight for XML file via pygments.

Manually tested with ReviewUI integration through Extensions patch: http://reviews.reviewboard.org/r/3565/
- Successfully installed this extension, its ReviewUI, and its Mimetype Handler
- Once installed, "Add Comment" button on XML attachments change to "Review" buttons.
- Once installed, able to review XML file attachments via the "Review" button.
- Once installed, XML file attachments now have thumbnails (if it was not enabled before)
- Color markup for the XML file attachment correctly rendered through pygments.